THE ABSOLUTTE STEREOCHEMICAL COUtRSE OF CITRIC ACID BIOSYNTHESIS* BY KENNETH R. HANSON AND IRWIN A. RoSEt DEPARTMENTS OF BIOCHEMISTRY OF THE CONNECTICUT AGRICULTURAL EXPERIMENT STATION AND OF YALE UNIVERSITY SCHOOL OF MEDICINE, NEW HAVEN Communicated by H. B. Vickery, September 3, 1963 In 1948 Ogston noted that when such symmetrical molecules as citric (or amino- malonic) acid are acted upon by asymmetric enzyme surfaces, pairs of identical groups may well be differentiated.1 Shortly thereafter it was shown, by using labeled substrates, that the two -CH2COOH groups of citric acid could, in fact, be distinguished.2 When citrate is formed from oxaloacetate and acetyl-CoA by citrate synthase (E.C.4.1.3.7, condensing enzyme), and is then treated with aco- nitate hydratase (E.C.4.2.1.3, aconitase) to form cis-aconitate and threo-Dr-iso- citrate, the -CH2COOH ends of these compounds are derived from acetyl-CoA, while the other atoms are derived either from oxaloacetate or water.3-6 The acetyl-CoA-derived group yields acetate when citrate is cleaved by citrate-lyase (E.C.4.1.3.6, citratase),7'8 and acetyl-CoA when citrate is attacked by the citrate cleavage enzyme.9 The nonequivalence of the two -CH2COOH groups is apparent when citric acid is represented as I (heavy line above, dotted lines below the plane of the paper). Carbon atoms 1 and 2 are to 5 and 4 as right arm is to left arm:'0 they cannot be superimposed except by reflection. The same molecule is shown in Emil Fischer projection as II (vertical lines below and horizontal lines above the plane of the paper). The carbon atoms are numbered according to the Hirschmann conven- tion.11 The major purpose of the present investigation has been to determine which -CH2COOH group, in the absolute stereochemical sense of I, is derived from acetate and which is derived from oxaloacetate. In order to differentiate the two groups it was necessary to obtain asymmetrically labeled citric acid from a labeled compound of known absolute configuration. The following procedure is based upon the investigations, by Dangschat and H. 0. L. Fischer, of the stereochemistry of quinic acid,'2 and by Davis and his associates of the biosynthesis of this compound.'3 In the first stage quinic acid labeled with tritium in the 6 position was obtained with the aid of an extract of Aerobacter aerogenes A170-143 known to contain the enzymes quinate dehydrogenase (E.C.1.1.1.24) and 5-dehydroquinate dehydratase (E.C.4.2.1.10).'3 Thus 5-dehydroshikimic acid (III) yielded 5-dehydroquinic acid- 6-T (IV), which on reduction gave quinic acid-6-T (V). The symbol,--, indicates that the configuration at the carbon atom bearing tritium is unknown. In the second stage V was oxidized to citric acid4-T (VII) by the method shown.'4 Lastly, the properties of (VII) as a substrate for aconitate hydratase were studied. Materials and Methods.-Enzyme preparations: Cells of A. aerogenes A170-143 were grown, with stirring at 370 for 24 hr, on minimal medium A15 supplemented with 20 mg of yeast extract (Difco) and 600 mg of quinic acid (volume 1 liter). Mucoid growth was not observed. The post log phase cells were washed with water, suspended in 0.033 M Tris-HC116 buffer, pH 7.5, and sonically disrupted. The initial extract (42 mg of protein from 1 ml of packed cells) showed strong NADH oxidase activity. After treatment with MnCl21' and dialysis against 0.033 M 981 Downloaded by guest on September 30, 2021 982 BIOCHEMISTRY: HANSON AND ROSE PROC. N. A. S. 5 4 OH 2 COOH 5 Tris-HCl buffer, pH 7.5, this activity was HOOC H2C T ,CH2COOH (A) H-C-H (P) 4 found to be negligible. The preparation vs,, (26 mg of protein) contained both quinate 'HUVU-C-OH dehydrogenase and 5-dehydroquinate de- COOH (P) H - -H tA) 2 hydratase. The specific activity of the COOH former was 1.06 units/mg of protein at pH 9.4.13 A small amount of 5-dehydroshiki- mic acid brought about the rapid oxidation FIG. 1. of NADH (pH 7.4, 0.72 units/mg of pro- tein). No appreciable losses of these ac- tivities occurred on storing the frozen solution at - 100 or on lyophylization. The A. aerogenes mutant was a gift of Dr. Bernard Davis. Aconitate hydratase was prepared from pig heart and stored as a frozen solution.'7 Before use it was activated with Fe2+ and cysteine. Isocitrate dehydrogenase (E.C.1.1.1.42) and glutamate dehydrogenase (E.C.1.4.1.3) in phosphate buffer were purchased from Boehringer. Throughout this paper a unit of enzyme is defined as that amount which catalyzes the trans- formation of 1 pmole of substrate/min at room temperature under optimum or specified conditions. Quinic acid-6-T: (a) From 5-dehydroshikimic acid: To the lyophylized enzymes (0.43 mg protein), buffer pH 7.4 (Tris-HCl, 100 stmoles in 33 ,1), 5-dehydroshikimic acid (8 jsmoles), tritiated water (0.25 ml), and, finally, NADH (13 ,umoles) were added and the reaction mixture left at 240. The oxidation of NADH was followed by diluting 5 jul aliquots to 1 ml with 0.1 M NaOH (added to terminate the reaction) and observing the decrease in absorbancy at 340 mMA. The oxidation was complete in 35 min, at which time further reaction was prevented by adding HCl (100 Mmoles). The tritiated water was removed by vacuum sublimation. Water was added and then removed by distillation, and this operation was repeated until all the easily exchangeable tritium had been displaced. The specific activity of the water in the reaction mixture was 578,000 cpm/Muatom. The 5-dehydroshikimic acid was a gift of Dr. P. R. Srinivasan. (b) From quinic acid: The enzymes (0.43 mg of lyophylized protein) in tritiated water (0.3 ml) were added to a lyophylized mixture of Tris-HCl buffer pH 7.4 (100 Moles), NAD + (1 MAmole), and potassium quinate (10 jMmoles). The reaction mixture was maintained at 240 for 2 hr and 370 for a further 6 hr. Samples were withdrawn at various times, and the extent of incorporation was estimated. The specific activity of the water in the reaction mixture was 1 X 106 cpm/ uatom. The reaction was terminated as in experiment (a). Citric acid-4-T by oxidation of quinic acid-6-T: A sample containing less than 10 Mmoles of tritiated quinic acid was chromatographed on a Dowex 1 acetate column." Measurements of radioactivity served to locate the quinic acid peak. The combined peak fractions were concen- trated to dryness, periodic acid (100 Mumoles, 0.1 ml) was added, and the solution left in the dark at 240 for 1 hr. Under these conditions quinic acid consumes exactly two molar proportions of periodate, the reaction being complete in about 10 min. Periodic and iodic acids were removed with the aid of a Dowex 1-X8 formate column (7 X 0.7 cm). The column was developed with 3 N formic acid and the first 16 ml of effluent were concentrated to dryness. Water (2 ml) and bromine (10 Ml) were added and the solution left in the dark at 27° overnight. The excess bromine was removed with a current of air and the reaction products were chromatographed on a Dowex-1 formate column.'8 The citric acid was located by radioactivity measurements. The over-all yield, based on radioactivity, was about 50%. The yield was not improved by adding strontium carbonate to the bromine oxidation mixture.'9 The specific activity of the citric acid from quinic acid was: method (a) 240,000 cpm/Mmole, and (b) 435,000 cpm/pmole. COOH HOs COOH HN0M COOH HO% COOH HON0 COOH nT x:) NbT4T/( T \O..HNf-OHC CH HOOC COOH ' ~~0 0 111 IV V VI VII FIG. 2. Downloaded by guest on September 30, 2021 VOL. 50, 1963 BIOCHEMISTRY: HANSON AND ROSE 983 The oxidation process was studied in greater detail. A sample containing quinic acid (176,000 cpm) labeled by method (b) was diluted with potassium quinate (250 Mmoles), and the solution chromatographed as above. The amount of acid in the various dried fractions was determined by titration, and 93%/O of the applied radioactivity was associated with the quinic acid peak. An isotope effect was observed: the specific activity fell linearly by 26% from the front to the tail of the peak; mean 617 epm/,umole. The sodium quinate fractions were combined and the solution was passed through a short Dowex 50-H+ column to give the free acid which was then oxidized. When the final reaction mixture was chromatographed, a series of peaks were recorded. The fourth and last peak corresponded to citric acid (33% yield). The specific activity of the acid increased by about 10% from the front to the tail of the peak. The mean specific activity (620 cpm/umole) was in close agreement with that of the quinic acid. Tritiated citric acids for control experiments: From isocitrate+T20: threo-D.-isocitric acid (from crassulacean plants) was treated with aconitate hydratase in the presence of tritiated water and the enzyme destroyed by heating. The residual isocitrate was converted to a-ketoglutarate with isocitrate dehydrogenase, and the mixture of acids separated by silica gel chromatography.20' Tritium was not incorporated into cis-aconitic acid. The incorporation into citric acid was 60% of the equilibrium value assuming labeling at a single position.2 Specific activity: 85,000 cpm/ Jzmole.